In the course of drilling an oil or gas well, the trajectory of the main well, or indeed a lateral well may intersect several independent formation pressure zones. Such zones may contain any combination of oil gas or water at different pressures, and as such have to be isolated from each other in order to control which zone is produced or not produced, and to prevent cross mixing between zones.
One method for achieving isolation is to deploy inflatable packers as part of the casing string and to inflate the packers, once they are in place, with cement pumped from the surface via special tooling that can be depth aligned with valves that allow the cement to enter into each independent packer. Although the pumping pressure is monitored at the surface, there are several potential leakage paths between the tool and the actual packer such that neither the volume nor pressure of the cement that enters the packer is known. If the packer is not adequately inflated and containment cannot be achieved, expensive rework or production difficulties may ensue.
Other than monitoring the actual pumping pressure or the volume of cement pumped, there is no attempt to monitor packer pressure during cementing operations.
In effect, permanent packers are inflatable systems which are inflated with cement pumped directly from the rig. A cementing tool with pressure or directional control cups is placed adjacent to the packer prior to pumping cement. The cups direct the cement via a check valve into the packer. The pumping pressure recorded at the surface together with the static head is assumed to be the pressure of the cement entering the packer. Improper positioning and leakage can significantly influence the packer pressure, but since there is no current instrumentation, the true value is never known.